Formation mechanism of shale in the first member of Lower Silurian Longmaxi Formation,Southeast of Central Sichuan Uplift
ZHAO Shaoze
XIE Hubin
LU Fengming
LI Yizhen
HAO Yuexiang
LI Yong
ZHAO Liang
XU Zheng
ZHANG Chenbo
LIU Yiqiang
Abstract：Notwithstanding the considerable potential exhibited by the shale in first member of Lower Silurian Longmaxi Formation(Long 1)in the Southeast of the Central Sichuan Uplift(SCSU),the exploration and development of shale gas remain hindered because of the ambiguous formation mecha-nism of the Long 1 shale.In this paper,the Long 1 in the SCSU of the Yangtze Plate is treated as the research object.Based on the data of core,logging,geochemistry and seismic profile,the study char-acterized the Long 1's sedimentary and organic matter enrichment,restored the sedimentary environ-ment,and finally discussed the shale formation mechanism according to the analysis on the genesis of gravity flow deposition.The Long 1 includes four lithologies:shale,silty shale,fine sandstone and medium sandstone.Shale can be further classified into siliceous shale,mixed-type shale and clay shale.Three gravity flow deposits,namely slumping deposits,sandy debris flow deposits and turbidi-ty flow deposits,were identified in the Long 1.The shale in the lower Long 1 under the first gravity flow deposition shows a high total organic carbon(TOC)mass fraction(wTOC),where the values of wV/wCr,wNi/wCo and wU/wTh are 1.47-4.68,4.09-9.69 and 0.49-1.31 respectively.Moreo-ver,the biological barium mass fraction w(Babio)is 294.09-993.29 μg/g;the wP/wAl value spans between 6.75× 10-3 and 29.37×10-3;the range of the chemical alteration index value(ICIA)is from 48.10 to 71.92;and the w(Al2O3)is 11.47％-16.44％.During the deposition of the lower Long 1,the bottom seawater of the study area was anoxic,accompanied by high marine productivity,and weak chemical weathering of the source area and the terrigenous input.In the upper Long 1 above the first gravity flow deposition,the values of wV/wCr,wNi/wCo and wU/wTh of the shale are 1.45-2.86,2.16-5.20 and 0.18-0.82,respectively.Besides,the w(Babio)ranges from 41.26 to 496.14μg/g;thewP/wAl value is 4.70×10-3-7.03×10-3;the ICIA is 70.41-75.77;and the span for w(Al2O3)is between 15.15％and 19.22％.During the deposition of the upper Long 1,the bottom seawater of the study area was oxygen-rich,alongside lower marine productivity,and stronger chemi-cal weathering of the source area and the terrigenous input.The research suggests that the gravity flow deposit of the Long 1 is a product derived from local tectonic activity and steepened slope that both caused by plate collision.Under the influence of the tectonic activity of CSU and global climate change,sedimentary environmental parameters,such as the redox conditions of bottom seawater,the seawater productivity,and the terrigenous input,jointly control the enrichment of organic matter in the Long 1 shale.The relative intensity variations of terrigenous input and upwelling lead to the dif-ferentiation of mineral components and lithofacies in the Long 1 shale.
Keywords：Southeast of Central Sichuan Upliftthe first member of Longmaxi Formationgravity flow depositionshaleformation mechanism
Publication Date：2025-07-30
Online Publishing Date：2025-08-15(First online date of this platform, not the publication date of the document)
Pages：18( 869-886 )